Fertilization of mouse eggs initiates a phenomenon termed "egg activation". Egg activation results in (1) the stimulation of cortical granule (CG) exocytosis that results in modifications of the zona pellucida (ZP) that constitute the block to polyspermy, and (2) the conversion of a meiotic cell cycle to a mitotic one (mitotic activation). By analogy to differentiated somatic cells, the sperm may initiate these events by a G protein-mediated signal transduction pathway that results in the generation of specific second messengers/ionic changes. The responses of somatic cells to humoral ligands that bind to cell surface receptors are generally confined to either exocytotic or mitogenic responses. What is unique about egg activation is that the sperm, which is analogous to a ligand, induces both an exocytotic and mitogenic response in the same target cell, i.e., the egg. It has been demonstrated that specific second messengers (inositol 1,4,5-trisphosphate) or activators of protein kinase C elicit the ZP modifications associated with egg activation, but do not result in mitotic activation. These observations suggest that although both CG exocytosis and mitotic activation are initiated by the fertilizing sperm, different second messenger systems may be activated that differentially regulate these two responses. Although G proteins mediate exocytosis and mitogenic activation in somatic cells, there are, in fact, no conclusive data that mammalian egg activation is mediated by a G protein-coupled signal transduction cascade. The experiments outlined in this proposal are designed to test the hypothesis that G proteins are involved in sperm- induced mammalian egg activation.